What does add_llvm_library exactly do? Why are libomp libraries installed twice?

I’m working on a patch to change the installation directory of openmp to lib/<target-triple> instead of lib/ (like other runtime libraries such as libunwind). I’ve noticed that files like libomptarget.rtl.amdgpu.so are installed by both a cmake install directly in /openmp/libomptarget/plugins/amdgpu/CMakeLists.txt and an install in add_llvm_library. My question is, why is this shared library installed twice, and what does add_llvm_library exactly do?